1. Field of the Invention
This invention relates to a method for increasing the lifetime of Stretford solution, more particularly to a method of preventing the formation of thiosulfate in the Stretford solution.
2. Description of the Related Art
Hydrogen sulfide is a contaminant present in many gas streams. Due to environmental regulations, hydrogen sulfide must normally be removed from gas streams before they are discharged. The presence of hydrogen sulfide can also prevent the use of the gas stream in other processes, because the hydrogen sulfide can deactivate the catalysts in these processes. Many methods have been developed to remove hydrogen sulfide from gas streams.
The Stretford process is one method for removing hydrogen sulfide from gas streams prior to discharge. The Stretford process employs an alkaline aqueous washing solution containing a water-soluble salt of anthraquinone disulfonic acid (ADA) and a water soluble vanadium compound such as sodium vanadate (NaVO3) to absorb the hydrogen sulfide from the gas stream and to oxidize the absorbed hydrogen sulfide to sulfur. The Stretford solution is regenerated by passing air through the solution.
The Stretford process is effective at substantially removing the hydrogen sulfide from gas streams, but maintaining of the Stretford solution significantly increases the operating costs. In particular, chemical reactions in solution result in a portion of the absorbed hydrogen sulfide and the product sulfur being converted to water-soluble sulfur-containing salts such as sulfates, thiosulfates, and polythionates. These salts increase the corrosivity of the washing solution, cause precipitation of components of the washing solution, and increase the rate of chemical consumption of the alkaline constituents of the washing solution. Eventually, the concentration of salts in the solution builds up to a level high enough that the salts must be removed from the solution or else it will not function properly to absorb hydrogen sulfide.
There are a number of methods for reducing the concentration of the sulfur-containing salts in Stretford solution. Some commercial plants periodically replace the entire washing solution and dispose the solution and the sulfur-containing salts. Other plants continuously bleed a slipstream of contaminated solution for disposal and add chemicals to replenish the chemicals in the circulating solution. Both disposal of the spent solution and the cost of the makeup chemicals greatly add to the cost of operating the Stretford plant.
There are a number of methods that have been developed to decrease the rate of buildup of salts in the solution and to prolong the life of the solution. The Unisulf process, described by Fenton et al. in U.S. Pat. No. 4,283,379, utilizes a washing solution comprising a soluble vanadium salt, a non-quinone aromatic compound, thiocyanate ions, and a water-soluble carboxylate complexing agent. The rate of formation of both sulfate and thiosulfate were greatly reduced compared to a conventional Stretford solution. Weber (U.S. Pat. No. 4,541,998) found that addition of thiocyanate to the washing solution suppressed the formation of thiosulfate. Although the lifetime of the solution is prolonged with these processes, sulfur-containing salts eventually build up in the solution, and the solution must be disposed.
Methods have also been developed to remove the salts from the Stretford solution. Farrington et al. (U.S. Pat. No. 4,360,508) describe a two stage method of desalting Stretford solution in which the solution is acidified with sulfuric acid to convert the sodium thiosulfate to sodium sulfate, sulfur dioxide, and sulfur through the following reactions:
Na2S2O3+H2SO4=Na2SO4+S+SO2+H2O
2 Na2S2O3+SO2=2 Na2SO4+3S
Overall:
3 Na2S2O3+H2SO4=3 Na2SO4+4S+H2O
The sodium sulfate is then removed from the solution by cooling, crystallizing the sodium sulfate as Glauber""s salt. The crystallized Glauber""s salt is separated from the solution by filtration, and the desalted Stretford solution is recycled to the Stretford unit.
Yan (U.S. Pat. No. 5,380,4420) describes a process in which the sodium thiosulfate is oxidized to sodium sulfate by catalytic oxidation. The sodium sulfate is removed by crystallization, and the desalted Stretford solution is reused.
Wolcott (U.S. Pat. No. 4,572,788) describes a method for treating spent Stretford solution in which a sidestream is taken from the solution, and ADA and vanadium are recovered from the sidestream by adsorbing the ADA onto charcoal and the vanadium onto an anion exchange resin. The ADA and the vanadium are recovered and are used to form new Stretford solution.
All of the methods of regenerating the Stretford solution are expensive and labor-intensive. There is a need for an improved and less expensive method of prolonging the lifetime of Stretford solution to reduce the cost of disposing or regenerating spent Stretford solution.
The present invention advantageously overcomes the shortcomings of the prior art by providing a process for prolonging the lifetime of Stretford solution by minimizing or eliminating the formation of thiosulfate salts in the Stretford solution.
In one aspect of the invention, there is provided a method for minimizing or eliminating the formation of thiosulfate salts in a washing solution for removing hydrogen sulfide from a gas stream by maintaining a concentration of sulfate salt in the solution at 100 grams/liter or less for an extended period of time. The concentration of sulfate salt is calculated on the basis of anhydrous sodium sulfate.
Preferably the extended period of time is at least as long as the length of time that would be required for the concentration of sulfate salt in the washing solution to reach 120 grams/liter, if sulfate salts are not removed from the solution.
In some embodiments, the method includes having thiocyanate ion in the Stretford solution. Preferably, the method also includes having a Stretford solution with less than about 5 grams/liter of thiosulfate ion. In a preferred embodiment, the Stretford solution contains between about 10 and 30 grams/liter thiocyanate ion. Advantageously, the sulfate salt is removed from the washing solution by cooling the solution.
In another aspect of the invention, there is provided a method for removing hydrogen sulfide from a hydrogen sulfide containing gas stream and converting the hydrogen sulfide to sulfur while forming substantially no thiosulfate. The method includes contacting the gas stream with a washing solution to absorb the hydrogen sulfide in the washing solution, allowing the washing solution to convert the hydrogen sulfide to sulfur, oxidatively regenerating the washing solution, recycling the regenerated washing solution into contact with the gas stream, and maintaining a concentration of 100 grams/liter or less sodium sulfate in the washing solution for an extended period of time.
Advantageously, the washing solution contains thiocyanate ion. Preferably, the washing solution contains less than about 5 grams/liter thiosulfate ion.
In yet another aspect of the invention, there is provided a method for minimizing or eliminating the formation of thiosulfate salts in Stretford solution. The method includes maintaining a concentration of less than 100 grams/liter sulfate salt in the Stretford solution for an extended period of time, where the concentration of sulfate salt is calculated on the basis of anhydrous sodium sulfate. Preferably, the Stretford solution contains at least about 15 grams/liter thiocyanate ion. Advantageously, the Stretford solution contains less than about 5 grams/liter thiosulfate ion.
In another aspect of the invention, there is provided a method for removing hydrogen sulfide from a gas stream and converting the hydrogen sulfide to sulfur while forming substantially no thiosulfate. The method includes contacting the gas stream with a washing solution having a pH between about 5 and 10. The washing solution contains a salt of anthraquinone disulfonic acid, water-soluble vanadium, and thiocyanate ion. The hydrogen sulfide is absorbed by the washing solution and is converted to sulfur by the washing solution. The washing solution is oxidatively regenerated and is recycled into contact with the gas stream. The method includes maintaining a concentration of sulfate salt in the washing solution at 100 grams/liter or less for an extended period of time, where the concentration of sulfate salt is calculated on the basis of anhydrous sodium sulfate. Preferably, there is less than about 5 grams/liter thiosulfate in the washing solution.